The present invention relates to a process for the preparation of 2,2,3,3-tetrafluoro-1,4-benzodioxanes, to novel intermediates occurring in this process, and to processes for their preparation. 2,2,3,3-Tetrafluoro-1,4-benzodioxanes are required as intermediates in the preparation of active compounds for pharmaceutics and agriculture (see, for example, DE-A37 16 652, U.S. Pat. No. 4,886,816, DE-A 19 501 367, DE-A4 415 435 U.S. Pat. No. 5,648,316, and DE-A4 217 725).
A three-step process starting from pyrocatechol for the preparation of unsubstituted 2,2,3,3-tetrafluoro-1,4-benzodioxane is known from the literature. In the first step, pyrocatechol is reacted with trifluorochloroethene to give 2,2,3-trifluoro-1,4-benzo-dioxane. The latter must then be chlorinated to give 2,2,3-trifluoro-3-chloro-1,4-benzo-dioxane. In the third step, finally, a chlorine/fluorine exchange is performed. This process has a series of disadvantages. Performance of the last step is only unsatisfactory. Fluorination by means of disproportioning in the presence of antimony(V) chloride is complicated from the technical point of view and requires 50% by weight of the material to be recirculated (see DE-A 3 315 147) U.S. Pat. No. 4,600,787. Reproducibility of the alternative gas-phase reaction with hydrogen fluorine at 350.degree. C. (see EP-A 623 609 U.S. Pat. No. 5,463,088) is poor, and its realization on an industrial scale would be extremely complicated. The corrosive media, such as the liberation of hydrogen chloride when chlorine gas is employed in the second step, and, if appropriate, the use of hydrogen fluorine in the third step, are of little advantage. Moreover, this process cannot be applied without difficulty to substituted pyrocatechols. For example, alkyl substituents on the benzene ring are also chlorinated under the chlorination conditions applied.